1. Field of the Invention
This invention relates to a disk for measuring a head position, which is used for testing or adjusting a displacement or shift quantity, that is, an off-track quantity of, a magnetic head from a desired track in a magnetic disk recording/reproducing apparatus (hereinafter designated as a disk apparatus) employing a magnetic disk, particularly a circular magnetic disk such as a floppy disk, and a measuring apparatus using the disk.
2. Description of the Prior Art
As a method of measuring the off-track quantity of the disk apparatus, an alignment disk designated as a cat's-eye disk (hereinafter denoted by CE disk) has been conventionally employed. FIG. 1 schematically shows the recorded pattern of the CE disk, and two single frequency data of A and B are recorded with O' as a center which is slightly spaced apart from a rotary center O of the disk. When a head H (tunnel erase head) is adapted to reproduce the data A and B at the center O of the disk under such a condition that the off-track is near zero, the waveform of eA=eB shown in FIG. 2(a) is obtained. The output voltage ratio of eA to eB corresponds to the off-track quantity of the head H. However, considerable error is generated in the measurement of the head position according to the conventional CE system. The cause depends upon a reproducing ability of erase gaps ER1 and ER2 of the head H. When data are recorded by a reading/writing (hereinafter denoted by R/W) head, the erase head essentially serves as the erasing function of both sides of the data by passing through DC current. However, in the case of the eccentrically recorded pattern such as CE data, the erase gaps ER1 and ER2 may pick up the magnetic fluxes from the data A and B since an erase core is arranged on the data, and a part of the magnetic flux may leak out to R/W core, resulting in an undesirable influence. FIG. 3 is one example showing a relation between a reproduced output in a usual track and the off-track quantity. The influence due to the reproduction of erase gaps will be clearly seen at portions of P and Q in FIG. 3. That is, the reproduced output approximately linearly decreases with increase of the off-track quantity, but the output does not become zero if the off-track quantity is made equal to the data width, and gradually decreases with a different slope to a point corresponding to the erase core width. The reproducing ability of the erase gap is below a few % against that of R/W gap in general tunnel erase heads. Accordingly, the reproduced output of the head H in FIG. 1 may become the combined output which is obtained from the output of R/W gap R and the output of erase gaps ER1, ER2, so that the output waveform will receive a kind of modulation by the phase difference between them. FIG. 2(b) is an enlarged view of a peak x in the waveform of FIG. 2(a). The phenomenon may be observed in the form of the noise and may be related to the error of the relative ratio of A' to B'.
As another conventional example, an alignment disk of a recorded pattern shown in FIG. 4 has been proposed (referring to Published Unexamined Patent Application No. 52-69309). In the drawing, 1 to 3 is a discrimination mark, and 4 to 9 is burst data which are alternately arranged at outer and inner circumference sides as compared with a track center N. The recorded pattern of the alignment disk is provided with 6 blocks, each of which is comprised of 1 to 9, within one track. FIG. 5 shows the waveforms of 1 to 9 which are reproduced by shifting the head H by x from the track center N in FIG. 4. In the drawing, the relative amplitude ratio eA/eB in 4 and 5, 6 and 7, 8 and 9 corresponds to the off-track quantity x. However, undesirable influence due to the reproducing ability of the erase head appears similarly even in this example, and the peak of respective waveforms is greatly disturbed (portion x in FIG. 5). Accordingly, the lowering of measurement accuracy will not be avoided by this example.